A piezoelectric thin film device is a device, which uses a bulk resonance of a piezoelectric film, for use as a filter or a resonator capable of activation in a certain frequency ranging from several hundreds Mega-Hertz to Giga-Hertz. Likewise, the resonator is used in a crystal oscillator; however, it is activated at lower frequency. When the piezoelectric thin film device is used in communication instruments for a mobile communication, for example, a frequency to be used is considerably high and ranges from several hundreds Mega-Hertz to Giga-Hertz.
For a device which uses a bulk resonance of the piezoelectric film, a resonance frequency of the film is mainly determined by a thickness of the piezoelectric film, so that the thickness is needed to be reduced to several micro-meters or less in order to resonate the film with frequency of Giga-Hertz. The crystal oscillator mentioned above is made by polishing a single crystal, thereby it is difficult to thin the oscillator up to about several micro-meters.
Recently, devices using the bulk resonance has been studied actively, in which piezoelectric thin films thinned up to about 1 to 2 .mu.m by various thin film fabrication method are typically used. Also, thin film electrodes formed on both sides of the piezoelectric thin film is applied with AC voltage for resonating the film. Also, in order to decrease an energy loss of a resonation, employed is a floating structure in which a portion positioned under resonant section of the film is partly etched.
One well-known method for fabricating the floating structure has fabricating an upper structure on a gallium arsenide (GaAs) substrate, etching the substrate from its bottom using an etchant such as sulfuric acid. For example, JP(A) 6-350154 discloses one fabrication method in which the floating structure is fabricated by forming an insulating film, a lower thin film electrode, a piezoelectric thin film, and an upper thin film electrode successively on surface of a substrate, and etching a part of the substrate from its opposite surface to the insulating film to form a floated resonant section on the etched part. The floating structure mentioned above can be fabricated using relatively simple processes.
However, the etched part or recess formed in the bottom surface of the substrate is closed at its top by the insulating film. Therefore, when the structure is mounted on a substrate with its bottom surface fixed to the substrate using an adhesive, the recess is disadvantageously closed. As a result, a pressure in the closed recess varies considerably at the vibration of the resonant section, which results in damages of the. device and a decrease in its reliability.
Also, a wire connected to the upper electrode is provided on the piezoelectric thin film and, in this instance, it should be extended over a step caused by the existence of the piezoelectric thin film, which may cause a disconnection of the wire and an increase of parasitic capacitance.
Also, the recess of the structure decrease the strength of the resonant section in particular as well as the yield of the device.